ROLE FOR A SYNAPSE-SPECIFIC CARBOHYDRATE IN AGRIN-INDUCED CLUSTERING OF ACETYLCHOLINE-RECEPTORS

Lectins such as VVA-B-4, which bind N-acetylgalactosaminyl (GalNAc)-te rminated saccharides, selectively stain the neuromuscular junction, th us defining a synapse-specific carbohydrate. In seeking roles for this carbohydrate, we asked whether VVA-B-4 affected the distribution of a cetylcholine receptors (AChRs) on cultured muscle cells. We found that incubation of myotubes with VVA-B-4 induced formation of AChR cluster s and potentiated the effect of a nerve-derived clustering factor, agr in. Additional experiments implicated GalNAc-terminated glycoconjugate s as modulators of agrin-induced AChR clustering. Enzymatic removal of GalNAc residues or treatment with a multivalent protein-GalNAc conjug ate blocked agrin-induced clustering, whereas enzymatic unmasking of a dditional GalNAc residues induced clustering in the absence of added a grin. Moreover, incubation with agrin led to redistribution of VVA-B-4 -binding material on myotubes. Together, these results suggest that ag rin-induced clustering of AChRs involves a GalNAc-dependent step.